This application generally relates to direct-write nanolithography instruments and cantilevered microactuators for use in, for example, direct-write printing methods and scanning probe lithography techniques, and methods of fabrication and use thereof.
U.S. Pat. No. 6,635,311 to Mirkin et al. (“Methods Utilizing Scanning Probe Microscope Tips And Products Therefor Or Produced Thereby”) discloses a direct-write patterning method in which, for example, a sharp tip (“pen”) coated with a chemical compound or mixture (“ink”) is contacted with a substrate, where it deposits said compound or mixture by capillary action. Arbitrary patterns may be fabricated with nanometer-scale resolution, and a wide variety of inks, including but not limited to biomolecules, metal or ceramic precursors, nanoparticles, and small organic molecules, can be used. Commercial applications are numerous. These include but are not limited to nanoencryption, the repair of photomasks used for semiconductor chip manufacturing, the repair of flat panel displays, such as computer and TV screens, and the fabrication of high-density biomolecule microarrays and nanoarrays.
The direct-write printing can be performed with an Atomic Force Microscope, which comprises a single microfabricated cantilever (usually a beam projecting from a handle substrate) with an integrated tip. In many cases, a laser beam is reflected on the back of the cantilever towards a quadrant photodiode to monitor its deflection, hence the force between its tip and sample. A feedback loop can be used to keep the cantilever deflection constant to track the sample topography during patterning.
Like e-beam lithography, the printing can be a serial pattern generation technique. It is thus commercially advantageous to operate multiple pens in parallel to increase the patterning throughput. These pens may be moved in concert by a macroscopic actuator (forming a “passive array”) or they may be individually actuated at least in one direction.
Passive pen arrays, which are commercially available from NanoInk, Inc. (Chicago, Ill.), can only produce a fixed number of copies of identical patterns. In contrast, individually actuated pens (also called “active pens”) may produce arbitrary patterns. In addition, individually actuated pens also provide: (a) the ability to write a pattern with a coated tip, then to image it with a clean tip, thus eliminating the sample contamination that would occur otherwise; (b) the ability to write with multiple inks in one writing session without the need to change probes; this permits the fabrication of multi-component nanopatterns; (c) an easier delivery of an ink to an individual probe, e.g. by dipping one probe at the time into microfluidic reservoirs (e.g., “inkwells”); and (d) the use of multiple tips coated with the same ink one after the next without need to recoat the tip, once ink is exhausted (the active pen array acting like an “Ink bandolier”).
U.S. Pat. No. 5,475,318 to Marcus et al. discloses a microfabricated, bimorph-actuated cantilever having a silicon tip and a resistive heater, which is intended for wafer probe cards. The method of preparation is very complex, thus prone to low yields and expensive. The articles do not address direct-write printing.
U.S. Pat. No. 6,642,129 to Liu et al. discloses individually actuated, thermomechanical or electrostatic probes for direct write printing from cantilevers and tips and a method of fabrication thereof. The patent describes preparing silicon nitride (SiNx) cantilevers and tips with a relatively simple process. SiNx cantilevers are preferred because of their higher resilience to brittle rupture than silicon ones. However, this process produces dull tips because it prepares convex tip molds (i.e., with their sharp end pointing away from the wafer substrate) and coats them with a (silicon nitride) film of finite thickness. Sharp tips are generally preferred, as higher resolution is achievable when patterning and imaging. The patent does not address some important issues with active pens. See, also, Zhang et al., Nanotechnology, 13, pp. 212-217, 2002; Bullen et al., Mat. Res. Soc. Proc., 758, pp. LL4.2.1-LL4.2.10, 2002.
U.S. Pat. No. 4,916,002 to Carver et al. at Stanford University (“Microcasting of microminiature tips”) discloses a method that uses anisotropically etched pits in silicon as a mold for tips. In this method, the tip is formed through and interlocked with an aperture in a cantilever. The tip fabrication step is distinct from the cantilever patterning; this process is meant to fabricate tips made of a different material than the cantilever.
U.S. Pat. No. 5,116,462 to Bartha et al., IBM, which is entitled “Method of producing micromechanical sensors for AFM/STM profilometry”, and U.S. Pat. Nos. 5,221,415 and 5,399,232 to Albrecht et al. (Stanford University) disclose a method of producing non-actuating cantilevers with integrated tips by (a) etching recesses in a (silicon) substrate through a (silicon dioxide) mask; anisotropic etching starting with square patterns results in inverted pyramids; (b) depositing a dielectric layer of e.g. silicon nitride using the recesses as tip molds; (c) patterning the dielectric layer and (d) releasing the cantilever so formed by etching the substrate away.
Further reduction of the tip apex radius may be achieved with the help of U.S. Pat. No. 5,580,827 to Akamine et al. (Stanford). '827 discloses a method of “casting sharpened microminiature tips” by submitting an etch pit in a silicon substrate to oxidation at a low temperature. Oxidation in these conditions is hindered at corner surfaces and thus forms a thinner layer near the bottom of the pit than on its sidewalls, leading to a sharpened tip after microcasting. Thus, it is another aspect to improve on Akamine's methods to fabricate active cantilevers with oxide-sharpened microcast tips.
Thus, a need exists to improve upon such methods to inexpensively fabricate actuated, including thermomechanically actuated, cantilevers with sharp tips and for the fabrication of passive (non-actuated) silicon nitride probes with integrated tips.
The use of concave etch mold comes at the expense of an additional wafer bonding step. Indeed, the handle substrate and the tip are preferentially placed on opposite faces of the cantilevered member to ensure that the tip, not the handle, contacts the sample first during lithography and imaging. However, the anodic bonding process that is disclosed in the aforementioned Albrecht and Bartha patents occurs at too high a temperature: annealing-induced stress in the metallic-thin-film bimorph element would result in uncontrolled cantilever curvature. Therefore, a need also exists to provide a method of fabrication for actuated cantilevers that does not require high-temperature processing of the wafer after metal deposition. In particular, it is an aspect to provide a method of fabrication of actuated cantilevers that utilizes low-temperature wafer bonding, and especially thermocompressive, eutectic or adhesive wafer bonding. Thermocompressive or eutectic wafer bonding may provide simultaneous electrical and mechanical connections.
A need also exists to provide a system that reversibly and reliably electrically connects a set of actuated cantilevers to their control electronics and mechanically tie them to a high-resolution actuator. It is commercially desirable to provide an actuated probe cartridge or flexible circuit (“flexcircuit”) that can be easily installed or replaced by the end user during operation.
It is yet another aspect to provide an instrument that comprises appropriate electronics and software to control the motion of actuated cantilever synchronously to the motion of a high-resolution actuator and permit their calibration and individual, manual or automated control for the purpose of nanolithography.
International PCT application WO 2004/044552 to Cruchon-Dupeyrat et al. (NanoInk) and WO 04/0022681 to Hantschel et al. (Palo Alto Research Center) entitled “Capillary-channel probes for liquid pickup, transportation and dispense using stressy metals” discloses methods and apparatuses to deliver multiple inks to cantilevers. A need exists to improve over current methods and apparatus to selectively deliver ink(s) to actuated cantilevers.
No admission is made that any of these references cited in the Background are indeed prior art.